The objective of our research is to relate the form and function of molecular assemblies in the cell. Coordinated X-ray diffraction and electron microscopy will be used in these investigations. Structural analysis of ordered aggregates of the muscle proteins produced in vitro reveal basic aspects of their organization in muscle. Crystals of the regulatory tropomyosin-troponin complex are being analyzed to determine the low resolution three-dimensional structure of tropomyosin and troponin, and changes with calcium. Attempts will be made to crystallize a number of other muscle proteins including troponin and its components, the S-l subfragment of myosin, and actin. A comparative study is in progress on the design of myosin-containing filaments. Structural analysis of microcrystals of a proteolytically modified fibrinogen is being carried out, and selective modifications of the molecule will be made to obtain other crystalline forms. Related studies on microtubule assemblies are also in progress. Comprehending the dynamics of all these systems requires clear images of their structures.